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The orbital angular momentum (OAM) of light, which is associated with the rotation of the phase structure, gives rise to numerous novel physical phenomena inlight–matter interactions. However, the direct generation of scalar OAM lasers with power‐handling capability and controllable states at the source remains challenging owing to mode degeneracy, handedness selection, and nonlinear thermal effects, which can even lead to topological charge splitting. This paper proposes a novel approach that allows spatial mode‐conversion‐controlled laser oscillation in a high‐gain OAM laser cavity with an integrated spiral‐face output coupler (SFOC). By adapting position‐dependent resonant modes, scalar optical vortices with a record power level (>60 W), high purity (>90%), and controllable OAM states (ħ or −8ħ) are demonstrated. Further, the dynamic process of OAM generation and evolution in the cavity is revealed. The concept presented herein opens a new route for the generation of OAM laser sources toward high power levels and holds promise for advancing classical and quantum optics, particularly in the area of light–matter interactions. [ABSTRACT FROM AUTHOR]

Intense femtosecond optical vortices with spatially structured amplitude and spiral phase front give rise to novel phenomena in light–matter interactions and strong‐field physics. However, current femtosecond vortex sources exhibit a poor power handling capability and amplification remains an open challenge due to a number of inherent technical difficulties. Here, it is demonstrated that a single‐crystal fiber laser amplifier is particularly well‐suited to directly amplify a femtosecond optical vortex without pulse stretching and compression in the time domain, while still maintaining the spatial properties associated with a clear central singularity and a spiral phase front, i.e., a well‐defined amount of orbital angular momentum (OAM).The optical nonlinearity experienced by such twisted light is verified to be substantially weaker compared to a fundamental mode beam where supercontinuum generation and spatial distortion are observed. The simple design and straightforward power scaling capability pave the way toward ultrahigh‐intensity femtosecond singular laser sources with an arbitrary topological charge. Such ultrafast OAM light sources are expected to help reveal complex physical phenomena in light–matter interactions and expand the applications to attoscience with X‐ray vortices, laser plasma acceleration, and micromachining. [ABSTRACT FROM AUTHOR]

The stability and accuracy of the on-orbit radiometric calibration of hyperspectral sensors are prerequisites for the quantitative application of satellite hyperspectral data. The Zhuhai-1 micro-nano satellite constellation is composed of eight hyperspectral satellite missions. The Orbita Hyperspectral Sensor (OHS) on board each satellite has a gradient filter spectroscopic design. When observing the Earth, eight integration stages can be set for each band according to different lighting conditions. Due to high manufacturing costs, OHSs are not equipped with on-board calibration devices. Therefore, it is very difficult to accurately calibrate OHSs for all of the integration stages. On the other hand, it is extremely important to ensure radiometric consistency between different OHSs within the Zhuhai-1 micro-nano satellite constellation. To carry out the rapid radiometric calibration of the Zhuhai-1 constellation, an on-orbit radiometric calibration model considering all of the integration stages related to hyperspectral sensors was built based on the BOA reflectance and atmosphere parameters published by the Committee on Earth Observation Satellites (CEOS) radiometric calibration network (RadCalNet). The RadCalNet product was used to derive the TOA radiance base in the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) radiative transfer (RT) model. In this paper, we analyzed the radiometric stability of the same sensor and the consistency of different calibration results regarding four RadCalNet sites, and the on-orbit radiometric performance evaluation of OHSs was also carried out. The data retrieved from OHSs regarding hyperspectral surface reflectance were preliminarily validated using site-synchronous surface reflectance measurements. [ABSTRACT FROM AUTHOR]

We report on a high-power Ho:YAG single-crystal fiber (SCF) laser inband pumped by a high-brightness Tm-fiber laser at 1908 nm. The Ho:YAG SCF grown by the micro-pulling-down technique exhibits a propagation loss of $0.05\pm 0.005~\text{cm}^{-1}$ at $2.09~\unicode[STIX]{x03BC}\text{m}$. A continuous-wave output power of 35.2 W is achieved with a slope efficiency of 42.7%, which is to the best of our knowledge the highest power ever reported from an SCF-based laser in the 2 $\unicode[STIX]{x03BC}\text{m}$ spectral range. [ABSTRACT FROM AUTHOR]

0.7 at.% Ho:Y 2 O 3 transparent ceramics were fabricated by using vacuum sintering plus hot isostatic pressing (HIPing) process. Vacuum sintered at 1500 °C for 12 h followed by HIPing at 1450 °C for 4 h, the Ho:Y 2 O 3 ceramics showed excellent in-line transmission and uniform grain size, with an average grain size of about 1 μm. The Ho:Y 2 O 3 ceramics showed good thermal conductivity thanks to the identical effective ionic radius between Ho 3+ and Y 3+ . For the first time, room temperature laser oscillation from Ho:Y 2 O 3 transparent ceramics was demonstrated. The uncoated 0.7 at.% Ho:Y 2 O 3 ceramic slabs were in-band pumped by a Tm:fiber laser at 1941 nm, and a maximum output power of 1.3 W at 2116.8 nm was achieved with a slope efficiency of 31.2%. It indicates that Ho:Y 2 O 3 transparent ceramics could be excellent laser gain medias for 2 μm solid-state laser applications. [ABSTRACT FROM AUTHOR]

Homogeneous and inhomogeneous spectrum broadening in Nd:Lu x Gd 1− x VO 4 series crystals were directly measured for the first time, and compared with that in the Nd:Lu x Y 1− x VO 4 and Nd:Gd x Y 1− x VO 4 crystals. Spectroscopic properties including fluorescence lifetime, spectral line width, emission peak, and stimulated emission cross section had obvious changes with the temperature increased from 77 K to 300 K, owing to the thermal phonon vibration. Compared to the pure vanadate crystals, we found these changes were more sensitive to temperature in the case of mixed crystals, in which cations (Lu, Y, and Gd) are random replaced by each other at the same lattice site. At 77 K, the thermal phonon vibration effect can be ignored and the inhomogeneous spectrum broadening that caused by disordered structure in all samplers can be calculated with the line-width of pure vanadate crystals as reference point, the corresponding values ( Δ Γ ( T ) ′ ) were 2.2 nm–3.8 nm in 4 F 3/2 → 4 I 11/2 transition and 1.7 nm–2.8 nm in 4 F 3/2 → 4 I 13/2 transition. Compared to Nd:Lu x Y 1− x VO 4 and Nd:Gd x Y 1− x VO 4 series mixed crystals, the change in spectral line width with x is most obvious in the case of Nd:Lu x Gd 1− x VO 4 series crystals, which may be attributed to the larger ionic radius difference between Gd and Lu ( R Lu < R Y < R Gd ). [ABSTRACT FROM AUTHOR]

An Er 3+ -doped Lu 1.5 Y 1.5 Al 5 O 12 (Er:LuYAG) solid-state laser with direct generation of optical vortex is reported. The vortex laser operation was realized through being pumped by an annular beam at 1532 nm, which was reformatted by a specially fabricated optical mirror. With two different laser output couplers of 10% and 20% transmissions, pure LG 01 mode lasers with right-handedness at 1647.7 nm and 1619.5 nm were yielded from a simple two-mirror cavity, respectively, without any helicity control optical element. Furthermore, stable pulse trains at 1647.7 nm have been achieved via employing an acousto-optic Q-switch, and ∼0.66 mJ pulsed energy and ∼65 ns pulse duration were finally obtained at 1 kHz repetition rate, corresponding to a peak power of ∼10.2 kW. The generated pulse vortex maintained LG 01 mode with well-determined right-handedness, as in the case of cw laser operation. [ABSTRACT FROM AUTHOR]

In this paper, we describe a thulium-doped (Lu,Y) 3 Al 5 O 12 mixed crystal that was designed and grown to achieve a large pulse energy in Q-switched laser operation. Its absorption cross-sections at 781.3 nm and 1629.2 nm were calculated to be 3.4 × 10 −21 cm 2 and 5 × 10 −21 cm 2 , respectively, and the emission cross-section at 2018.3 nm was calculated to be 1.92 × 10 −21 cm 2 . The highest single pulse energy of 1.4 mJ near 2.02 μm was realized in an actively Q-switched solid-state laser, and further power scaling was just limited by the damage of the crystal coating. Based on the calculated spectrum parameters, theoretical simulation of the single pulse energy that considering the cross-relaxation process was proposed, which was well consistent with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: Pb nanoparticle tipped ZnO nanowires heterostructures (i.e., Pb/ZnO heterostructures) were successfully prepared by long-pulse-width laser ablation of Zn/Pb binary metal target in gas mixture of Ar/O2. Transmission electron microscopy (TEM) and selected area electron diffraction were employed to characterize the morphology and structure of the/ZnO heterostructures. Furthermore, some special hierarchical structures were also observed in the as-prepared samples, which consists of smaller Pb nanoparticles tipped shorter ZnO nanowires heterostructures with almost array perpendicularly grow on the two sides of one thick ZnO nanowire. The experimental results show that the formation of Pb/ZnO heterostructures was influenced by the pulse width. The present study provides new insights into the research on the fundamental aspects of laser ablation in gas atmosphere. [Copyright &y& Elsevier]

Robust calibration and validation (Cal and Val) should guarantee the accuracy of the retrieved information, make the remote sensing data consistent and traceable, and maintain the sensor performance during the operational phase. The DRAGON program has set up many remote sensing research topics on various application domains. In order to promote the effectiveness of data modeling and interpretation, it is necessary to solve various challenges in Cal and Val for quantitative RS applications. This project in the DRAGON 4 program aims to promote the cooperation of the Cal and Val experts from European and Chinese institutes in Cal and Val activities, and several achievements have been obtained in the advanced on-orbit optical sensor calibration, as well as microwave remote sensor calibration and product generation. The outcomes of the project have benefited the related remote sensing modeling and product retrieval, and promoted the radiometric calibration network (RadCalNet) as an international operational network for calibration, intercalibration, and validation. Moreover, this project provided local governments with a more accurate OMI NO2 data in China, which were used to study the air quality control during APEC period, Parade period and G20 period. This will be of ongoing be value for monitoring atmospheric environmental quality and formulating pollution control strategies. [ABSTRACT FROM AUTHOR]

Vicarious calibration and validation techniques are important tools to ensure the long-term stability and inter-sensor consistency of satellite sensors making observations in the solar-reflective spectral domain. Automated test sites, which have continuous in situ monitoring of both ground reflectance and atmospheric conditions, can greatly increase the match-up possibilities for a wide range of space agency and commercial sensors. The Baotou calibration and validation test site in China provides operational high-accuracy and high-stability vicarious calibration and validation for high spatial resolution solar-reflective remote-sensing sensors. Two sites, given the abbreviations BTCN (an artificial site) and BSCN (a natural sandy site), have been selected as reference sites for the Committee on Earth Observation Satellites radiometric calibration network (RadCalNet). RadCalNet requires sites to provide data in a consistent format but does not specify the required operational conditions for a RadCalNet site. The two Baotou sites are the only sites to date that make spectral measurements for their continuous operation. One of the core principles of RadCalNet is that each site should have a metrologically rigorous uncertainty budget which also describes the site's traceability to the international system of units, the SI. This paper shows a formalized metrological approach to determining and documenting the uncertainty budget and traceability of a RadCalNet site. This approach follows the Guide to the Expression of Uncertainty in Measurement. The paper describes the uncertainty analysis for bottom-of-atmosphere and top-of-atmosphere reflectance in the spectral region from 400 to 1000 nm for the Baotou sites and gives preliminary results for the uncertainty propagating this to top-of-atmosphere reflectance. [ABSTRACT FROM AUTHOR]

The advanced hyperspectral imager (AHSI) is one of the sensors aboard the Chinese Gaofen-5 (GF-5) satellite, possessing characteristics of high spatial and spectral resolution, as well as width swath. To better understand the radiometric performance of GF-5/AHIS after its launch, this article presents an on-orbit radiometric calibration approach for AHSI visible and near-infrared (VNIR) and shortwave infrared (SWIR) sensors from field automatic observations with a field spectrometer in the absence of SWIR measurements. A spectrum extension method was proposed to extend the retrieved surface hyperspectral reflectance in the VNIR spectral ranges to SWIR by incorporating the historical hyperspectral reflectance library. The radiometric calibration coefficients of GF-5/AHSI were calculated by linear fitting of the observed digital number (DN) values with GF-5/AHSI and predicted at-sensor radiances with MODTRAN 5 based on extended hyperspectral surface reflectance. Comparisons with onboard calibration results were also performed, and the averaged relative differences were within 5% with $1\delta $ standard deviations less than 10% for most bands, except for those in the atmospheric absorption and low signal-to-noise ratio bands. The comparison results indicate that the on-site radiometric calibration results are consistent with the onboard results, and the operational on-orbit radiometric calibration approach is reliable in the case that there are no measurements in the SWIR spectra range. The on-orbit radiometric performance of GF-5/AHSI rapidly degraded during the first several months after its launch and then tended to be relatively stable. [ABSTRACT FROM AUTHOR]

We report on a power-scalable sub-100-fs laser in the 2-μm spectral range using a Tm3+-doped 'mixed' (Lu,Sc)2O3 sesquioxide ceramic as an active medium. Pulses as short as 58 fs at 2076 nm with an average output power of 114 mW at a pulse repetition rate of approximately 82.9 MHz are generated by employing single-walled carbon nanotubes as a saturable absorber. A higher average power of 350 mW at 2075 nm is obtained at the expense of the pulse duration (65 fs). A maximum average power of 486 mW is achieved for a pulse duration of 98 fs and an optical conversion efficiency of 22.3%, representing the highest value ever reported from sub-100-fs mode-locked Tm lasers. [ABSTRACT FROM AUTHOR]

PPA is found to be an efficient catalyst for the synthesis of pyrimidinones (IV) via the Biginelli reaction. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a SESAM mode-locked Tm:CaYLuAlO4 (a-cut) laser in the 2-μm spectral range. A birefringent filter (BF) is employed to shift the emission wavelength above 2 μm to support the stable mode locking through avoiding the structured water vapor absorption. Pulses as short as 288 fs are generated with a central wavelength of 2037 nm and an average power of 166 mW at a repetition rate of ~ 77.6 MHz. Moreover, wavelength tunable femtosecond laser with a tuning range from 2031.8 to 2081.1 nm is realized simply by rotating the birefringent filter around its surface normal. This work experimentally shows that birefringent filter is a simple and low-cost way to support the stable mode locking of Tm-bulk femtosecond lasers with an emission peak in the water vapor absorption region. [ABSTRACT FROM AUTHOR]

We demonstrate the direct generation of optical vortex pulses with stable energy and changeable orbital angular momentum. Single Laguerre-Gaussian (LG0,l) laser modes were directly generated using a laser diode with output intensity profile of doughnut distribution. With passive Q-switching, vortex pulses with stable energy were obtained. Moreover, the topological charge was changeable by variation of the pump power. By a mode-converter and second harmonic generation, the LG0,2l mode was identified. It can be proposed that this pulsed laser should have promising applications in various fields based on its compact structure, stable and high pulse energy, and changeable orbital angular momentum. [ABSTRACT FROM AUTHOR]

Here, we demonstrate a broadband tunable (Lu0.0541Tm0.0548Gd0.8911)3Ga3O12 (Tm:LuGGG) laser in-band pumped by a homemade 1645-nm Er:YAG laser. A maximum average output power of 0.7 W at 1997 nm is obtained with a slope efficiency of 46.3%. We also demonstrate a continuously tuning output ranging from1854 to 2066 nm with a tuning coverage of 212 nm. The results indicate that the Tm:LuGGG crystal is an excellent laser medium for high-efficient broadband light generation, which may help to explore potential applications in ultrafast lasers. [ABSTRACT FROM AUTHOR]

Highlights: [•] The 4 F 3/2 → 4 I 13/2 transition of Nd:La0.11Y0.89VO4 was investigated for first time. [•] The fluorescence peak of crystal exhibited obvious inhomogeneous broadening. [•] Continuous-wave and active Q-switched laser operations were realized. [•] The relationship of absorbed pump power and output power was depicted. [•] The shortest pulse width, highest peak power reached 13ns, 2.69kW, respectively. [ABSTRACT FROM AUTHOR]

For the first time, graphene saturable absorber (SA) is used as the Q -switcher for a 946 nm Nd:YAG laser. Using a 5–7 layer graphene SA with initial transmission of 78% and an output coupler with transmission of 5%, efficient Q -switching operation is realized. When the absorbed pump power is raised to 9.5 W, a maximum output power of 0.9 W is obtained, corresponding to an optical conversion efficiency of 9.5%. The pulse repetition frequency and the pulse width are 225 kHz, 162 ns, respectively, resulting a pulse energy of 4 μJ and a peak power of 24.7 W. With a BiB 3 O 6 (BIBO) crystal as the frequency doubler, 473 nm blue laser is generated. The present research manifests graphene has great potential for short wavelength laser modulation. [ABSTRACT FROM AUTHOR]

Highlights • Our work firstly and systematically discussed the anisotropy of pulsed laser properties in monoclinic, disordered crystal Nd:LYSO with three different orientation (x-, y- and c-cut). • By analyzing the stimulated emission cross-section and the experimental results, Nd:LYSO of three orientation performs different properties. This has guiding significance for different applications by choosing different crystal. • The Y transmission direction has much larger pump absorbing efficiency and the maximum pulse energy and the highest peak power were 150.8 μJ and 14.7 kW, respectively. • The X transmission direction is the most preferred for multi-wavelength operation, which could be adopted to be a multi- wavelength laser source to generate coherent THz waves by difference frequency. Abstract We report on a Cr:YAG Q-switched laser using monoclinic, disordered Nd:LYSO crystal as gain medium. Anisotropy in pulsed laser performances is investigated with samples in different cut directions. With Y-cut Nd:LYSO, pulse energy of 150.8 μJ and peak power of 14.7 kW are achieved at 1079 nm. However, in the case of X-cut Nd:LYSO, multi-wavelength emission at 1076, 1077.5 and 1079 nm are observed, the total pulse energy and peak power are 120.9 μJ and 7.8 kW respectively. [ABSTRACT FROM AUTHOR]

• Spectral properties and high-efficiency broadband laser operation of Tm:CYGA crystal at 2 μm were demonstrated. • The maximum continuous-wave output power of 1.43 W was obtained. • A 284.7 nm tuning coverage was realized with a BF. We report on spectral properties and a high-efficiency broadband laser operation of Tm:CaY 0.9 Gd 0.1 AlO 4 (Tm:CYGA) crystal. The polarized absorption and emission spectra, gain cross sections and fluorescence lifetime are systematically studied, presenting a broad absorption and emission spectra and thus a smooth gain band. Furthermore, under an absorbed pump power of 4.35 W, a maximum continuous-wave (CW) laser output power of 1.43 W at 1967.8 nm is obtained, corresponding to a slope efficiency of 42.2 %. The beam quality factors (M2) in X-axis and Y-axis are measured to be 1.011 and 1.027, respectively. A tunable Tm:CYGA laser with a tuning range of 284.7 nm from 1837.5 to 2122.2 nm is achieved employing a birefringent filter (BF). The results indicate that the Tm:CYGA crystal is an excellent laser medium for generating high-efficiency broadband CW laser, indicating its potential for ultrashort pulse generation in the 2-μm spectral range. [ABSTRACT FROM AUTHOR]

Tm3+,Ho3+-codoped and compositionally "mixed" disordered Ca(Ga,Lu)AlO 4 aluminate crystals are promising materials for ultrashort pulse generation above 2 μm. Their polarized spectroscopic properties are studied and the effect of Lu3+ codoping on the inhomogeneous spectral broadening is verified. The peak stimulated-emission cross-section for the 5I 7 → 5I 8 Ho3+ transition is 1.05 × 10−20 cm2 at 2000 nm for π-polarization. The Tm3+ ↔ Ho3+ energy-transfer parameters are P 28 = 1.42 and P 71 = 0.090 [10−22 cm3μs−1] indicating a direct transfer with a thermal equilibrium time of 4.35 ms. The effective gain spectra for the codoped crystal are derived. For σ-polarization, the gain bandwidth at ∼2.04 μm is about 100 nm; particularly for this light polarization, Lu3+ codoping induces a noticeable red-shift of the emission spectra further beyond 2 μm. Using absorption studies at 12 K, the Stark splitting of the Tm3+ and Ho3+ multiplets is resolved. A laser-pumped Tm,Ho:Ca(Ga,Lu)AlO 4 laser generated a maximum output power of 763 mW at 2078.6 nm with a slope efficiency of 26.4% and π-polarized emission. A continuous wavelength tuning between 1887.7 and 2127 nm (tuning range: 239.3 nm) was achieved for σ -polarization. Diode-pumped operation of a -cut and c -cut crystals was also studied. • Tm3+,Ho3+-codoped "mixed" disordered Ca(Ga,Lu)AlO 4 aluminate crystal. • The effect of Lu3+ codoping on the inhomogeneous spectral broadening is verified. • For σ-polarization, the gain bandwidth at ∼2.04 μm is about 100 nm. • The Tm,Ho:Ca(Ga,Lu)AlO 4 laser generated 763 mW at 2078.6 nm in π-polarization. • A continuous wavelength tuning between 1887.7 and 2127 nm was achieved. [ABSTRACT FROM AUTHOR]

Ho3+-doped CaY 0.9 Gd 0.1 AlO 4 single crystal was successfully grown by the Czochralski technique for the first time. The polarized absorption spectra, polarized luminescence spectra and fluorescence decay curve of the crystal were investigated at room temperature. The spectroscopic parameters are calculated based on Judd–Ofelt theory, the values of Ω 2,4,6 were calculated to be 3.16 × 10−20 cm2, 3.93 × 10−20 cm2, 0.91 × 10−20 cm2, respectively. The Ho:CaY 0.9 Gd 0.1 AlO 4 crystal exhibits strong emission at 2013 nm for π- and 2032 nm for σ-polarizations with stimulated emission cross section of 6.64 × 10−20 cm2 and 1.90 × 10−20 cm2, respectively. The fluorescence lifetime of the 5I 7 level was measured to be 6.60 ms. The continuous-wave (CW) laser operation of Ho:CaY 0.9 Gd 0.1 AlO 4 crystal around 2 μm was demonstrated. • Ho3+-doped CaY 0.9 Gd 0.1 AlO 4 single crystal was successfully grown by the Czochralski technique for the first time. • The emission cross-section at 2013 nm and 2032 nm are 6.64 × 10−20 cm2 and 1.90 × 10−20 cm2 for π- and σ-polarizations respectively. • The continuous-wave (CW) laser operation of Ho:CaY 0.9 Gd 0.1 AlO 4 crystal around 2 μm was demonstrated. [ABSTRACT FROM AUTHOR]

"Mixed" calcium rare-earth aluminate laser host crystals Ca(Gd,Lu)AlO 4 (CALGLO) with up to 10.8 at.% Lu codoped with Tm3+ and Ho3+ ions are grown by the Czochralski method along the [001] direction. The segregation of rare-earth ions is studied. The crystal structure is refined by the Rietveld method. Tm,Ho:Ca(Gd,Lu)AlO 4 crystallizes in the tetragonal system (sp. gr. I 4/ mmm) exhibiting a K 2 NiF 4 type structure. The lattice constants are a = 3.6585(6) Å and c = 11.9660(9) Å for a crystal with a composition of CaGd 0.8947 Lu 0.0551 Tm 0.0448 Ho 0.0054 AlO 4. The stability of Ca(Gd,Lu)AlO 4 solid-solutions is discussed. The polarized Raman spectra are measured, revealing a most intense mode at 311 cm−1 and a maximum phonon frequency of ∼650 cm−1. The polarized absorption spectra are measured. The transition intensities for the Ho3+ ion are analyzed using the modified Judd-Ofelt theory accounting for configuration interaction. • Tm3+,Ho3+-codoped "mixed" disordered Ca(Ga,Lu)AlO 4 aluminate crystal. • Rietveld refinement of the crystal structure and discussion of the crystal disorder. • The stability of CaGd 1-x Lu x AlO 4 solid-solutions is discussed. • Polarized Raman spectra are measured, revealing the most intense mode at 311 cm−1. • Transition intensities for Ho3+ ions are analyzed using the modified Judd-Ofelt theory. [ABSTRACT FROM AUTHOR]

Nonlinear optical crystals of LaCa4O(BO3)3 (LaCOB) were grown by the Czochralski method. Crystal cracks induced by cleavage along the (2̅01) planes were observed and studied with respect to the crystal structure. Efficient second-harmonic-generation (SHG) was realized in the LaCOB crystals for the first time. For a (15°, 180°)-cut with the dimensions of 3 × 3 × 8 mm3, the SHG conversion efficiency of a 1064 nm Nd:YAG laser reached 49%. [ABSTRACT FROM AUTHOR]

Abstract: The continuous-wave (CW) and passively Q-switched laser performance of a Nd:Gd0.63Y0.37VO4 microchip laser are reported. To the best of our knowledge, this is the first time that a mixed vanadate crystal is used as the gain medium in an LD-pumped microchip laser. When the incident pump power was 6.16W, the CW laser output was 2.09W, corresponding to an optical conversion efficiency of 33.9% and a slope efficiency of 37.9%. Using a Cr4+:YAG crystal as the saturable absorber as well as the output coupler, the pulse width, pulse energy, and peak power were 1.17ns, 39.5μJ, and 33.8kW, respectively. [Copyright &y& Elsevier]

We report on the spectropic and continuous-wave (CW) laser properties of a novel Er3+-doped mixed garnet (Er:LuYSGG) crystal. The maximum phonon energy of Er:LuYSGG was measured to be 720 cm−1, which is lower than that of Er:YSGG crystal. Fluorescence lifetimes of 4I 11/2 and 4I 13/2 levels amounts to 2.1 ms and 3.2 ms, indicating a potential capability for mid-infrared laser emission. For the CW laser operation, a maximum output power of 472 mW was achieved at 2797 nm, with a slope efficiency of as high as 24%. • An Er3+-doped (Lu, Y, Sc)-mixed garnet crystal that has a long fluorescence lifetime (4I 11/2 : >2 ms) is proposed. • The phonon energy of Er:LuYSGG crystal is further reduced compared with that of Er:YSGG. • CW laser performance of Er:LuYSGG crystal gives a slope efficiency of 24%, confriming the feasibility of further power improvement. [ABSTRACT FROM AUTHOR]

We report on the growth, structure refinement, optical spectroscopy, continuous wave and femtosecond mode-locked laser operation of a Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. The 2.64 at.% Tm, 0.55 at.% Ho:CNGG is grown by the Czochralski method. Its cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method revealing a random distribution of Ga3+ and Nb5+ cations over octahedral and tetrahedral sites. The Ho3+ transition probabilities are determined within the Judd-Ofelt theory accounting for an intermediate configuration interaction (ICI). For the 5I 7 → 5I 8 Ho3+ transition, the maximum stimulated-emission cross-section σ SE is 0.47 × 10−20 cm2 at 2080.7 nm. The gain bandwidth of Tm,Ho:CNGG at ∼2 μm is > 150 nm and the thermal equilibrium decay time - 6.80 ms. The Tm3+ ↔ Ho3+ energy transfer parameters are determined. A diode-pumped Tm,Ho:CNGG microchip laser generated 413 mW at 2088.4 nm with a slope efficiency of 15.9%. A continuous wavelength tuning between 1940.3 and 2144.6 nm is demonstrated. Ultrashort pulses as short as 73 fs are achieved at 2061 nm from a Tm,Ho:CNGG laser mode-locked by a GaSb semiconductor saturable absorber mirror at a repetition rate of 89.3 MHz. • Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. • Cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method. • Transition probabilities, emission cross-section and Tm3+↔Ho3+ energy-transfer. • Continuous-wave Tm,Ho:CNGG laser is tuned between 1940.3 and 2144.6 nm. • Tm,Ho:CNGG laser is mode-locked by a GaSb-based SESAM generating 73 fs pulses. [ABSTRACT FROM AUTHOR]

The Day–Night Band (DNB) imaging sensor of the Visible Infrared Imaging Radiometer Suite (VIIRS) adds nighttime monitoring capability to the Suomi National Polar-Orbiting Partnership and National Oceanic and Atmospheric Administration 20 weather satellite launched in 2011 and 2017, respectively. Nighttime visible imagery has already found diverse applications, but image quality is often unsatisfactory. In this study, variations in observed top-of-atmosphere (TOA) reflectance were examined in terms of nighttime bidirectional effects. The Antarctica Dome C ground site was selected due to high uniformity. First, variation of reflectance was characterized in terms of viewing zenith angle, lunar zenith angle, and relative lunar azimuth angle, using DNB data from 2012 to 2020 and Miller–Turner 2009 simulations. Variations in reflectance were observed to be strongly anisotropic, suggesting the presence of bidirectional effects. Then, based on this finding, three popular bidirectional reflectance distribution function (BRDF) models were evaluated for effectiveness in correcting for these effects on the nighttime images. The observed radiance of VIIRS DNB was compared with the simulated radiance respectively based on the three BRDF models under the same geometry. Compared with the RossThick-LiSparseReciprocal (RossLi) BRDF model and Hudson model, the Warren model has a higher correlation coefficient (0.9899–0.9945) and a lower root-mean-square-error (0.0383–0.0487). Moreover, the RossLi BRDF model and Hudson model may have similar effects in the description of the nighttime TOA over Dome C. These findings are potentially useful to evaluate the radiometric calibration stability and consistency of nighttime satellite sensors. [ABSTRACT FROM AUTHOR]

Field calibration is a feasible way to evaluate space-borne optical sensor observations via natural or artificial sites on Earth's surface with the aid of synchronous surface and atmospheric characteristic data. Since field calibration is affected by the coupled effects of surface and atmospheric characteristics, the single calibration results acquired under different surface and atmospheric conditions have different biases and different uncertainties, making it difficult to determine the consistency of these multiple calibration results. In view of this, by assuming that the radiometric performance is invariant during field calibration and the calibration samples are independent of each other, the surface–atmosphere invariant Key Comparison Reference Value (KCRV) is essentially derived from various calibration results. As the number of calibration samples increases, the uncertainty in the KCRV should decrease, and the KCRV should approach the "true" value. This paper addresses a novel method for estimating a weighted average value from multiple calibration results that can be used to compare each calibration result, and this value is accepted as the KCRV. Furthermore, this method is preliminarily applied to the field calibration of the Multispectral Instrument (MSI) onboard the Sentinel-2B satellite via the desert target at the Baotou site, China. After employing a chi-squared test to verify that 12 calibration samples are independent from each other, the KCRV of the 12 calibration samples at the Baotou site is derived, which exhibits much lower uncertainty than a single sample. The results show that the KCRVs of the relative differences between the simulated and observed at-sensor reflectance are 3.75%, 5.11%, 6.09%, and 5.03% for the four bands of Sentinel-2B/MSI, respectively, and the corresponding uncertainties are 1.84%, 1.87%, 1.90%, and 1.93%. It is noted that the KCRV uncertainty obtained with only 12 calibration samples is reduced significantly, and in the future, more samples in other instrumented sites will be used to validate this method thoroughly. [ABSTRACT FROM AUTHOR]